This invention relates to carbon fibers excellent in substrate strength and adhesion to a matrix resin and a method for producing same.
Carbon fibers which have been used as reinforcing materials for composite materials are not necessarily sufficient in adhesion to matrix resins and so the surface of the carbon fibers must be activated. For this purpose, there have been employed various surface treatments such as treatments with chemical agents, vapor phase oxidation, electrolytic oxidation, etc. Among these treatments, the electrolytic oxidation is a practical surface treatment because of good operability, easiness in control of the activating reaction, saving of energy, etc.
The conventional surface treatment technique have necessarily damaged carbon fibers to cause reduction of substrate strength and thus surface treatment effect cannot be sufficiently exhibited.
Hitherto, interlaminar shear strength (referred to as "ILSS" hereinafter) has been used as a means to evaluate the adhesion strength between carbon fibers and matrix resins which constitute a composite material. However, ILSS is not so sensitive for obtaining a measure of the uneven surface treatment and other conditions of the treatment in the case of composite materials composed of carbon fibers which have been subjected to a surface treatment of more than a certain level and does not directly express the adhesion strength between the carbon fibers and the matrix resins.
On the other hand, the tensile strength in the direction perpendicular to the orientation direction of fibers which shows the peel strength between carbon fibers and matrix resin which constitute a composite material (said tensile strength is referred to as "TS.perp." hereinafter) does sensitively reflect the degree of surface treatment, unevenness of surface treatment and other effects brought about by the change of treatment conditions and is a characteristic value very important for design of composite laminate materials. However, at present, TS.perp. of composite materials containing carbon fibers subjected to the conventional surface treatment has not yet reached a practical level.
When carbon fibers are subjected to surface treatment in order to improve the adhesion strength between the carbon fibers (reinforcing material) and matrix resin, use of the conventional surface treatment technique often damages the carbon fibers to cause reduction of substrate strength and thus effect of the surface treatment cannot be sufficiently exhibited.
For improvement of the surface treatment of carbon fibers, there have been proposed regulation of crystalline orientation by electron diffraction method and regulation of oxygen concentration measured by X-ray photoelectron spectroscopy (ESCA method) both for controlling amount of surface functional groups (cf. Japanese Patent Unexamined Publication (kokai) No. 214527/83). Furthermore, there has been proposed introduction of nitrogen-containing functional group by electrolytic reduction in an electrolyte containing amine (cf. Japanese Patent Unexamined Publication (Kokai) No. 112068/84). However, none of these methods have given sufficient effects.
The surface of carbon fibers carbonized by common methods have fragile portions because the fibers undergo various chemical and mechanical damages such as rapid cooling, heating and elongation during the carbonization or friction with or pressing by a roller portion. Most of these fragile portions comprise carbon material of relatively low crystallinity and irregular structure and these portions are often starting points for tensile breaking of strands of the carbon fibers. Thus, it is effective for improvement of strand strength to eliminate the fragile portions. Further, these fragile portions are weak in bonding to carbon fiber substrate and are in easily peeling state. Therefore, TS.perp. of composite materials made using carbon fibers which have been subjected to surface treatment without eliminating the fragile portions is considered to be influenced by peeling of the fragile portions from the substrate and effect of surface treatment is not sufficiently exhibited. In fact, observation of a rupture cross-section of a test piece of a composite material reinforced with carbon fibers after subjected to a test for TS.perp. by an electron microscope reveals that there are many portions where peeling occurs in inner part of fibers near the surface layer. That is, in order to improve substrate strength of carbon fibers and peel strength of carbon fibers-reinforced composite material, it is necessary to remove fragile portions of the surface of carbon fibers without producing new defective portions to expose the surface free of defects.
As a result of the inventors' investigation of causes for the low TS.perp. of the conventional carbon fiber-reinforced composite materials, it has been found that there are fragile portions in the surface part of carbon fibers which weakly bond to fiber substrate and these fragile portions are causes for the fact that the characteristics of carbon fibers such as strength and elastic modulus of carbon fibers cannot be reflected in the composite materials and that amount of nitrogen in the inner part and amounts of nitrogen and oxygen functional groups in the surface part and amount of silicon oxide in the surface part are closely related with adhesion strength and substrate strength and especially, control of the amount of the surface nitrogen is essential for improvement of adhesion strength and substrate strength.